The present invention relates to an apparatus and a method for taking radiologic transmitted images of an arbitrary object from a plurality of directions and reconstructing a radiologic three-dimensional image of the object on the basis of the transmitted images, or more in particular to an apparatus and a method for taking radiologic three-dimensional images which is capable of producing an X-ray three-dimensional image containing information far greater in amount and a medical diagnostic capability higher than the conventional X-ray tomographic image or the conventional X-ray transmitted image obtained by digital radiography.
Conventional radiologic imaging apparatuses include the X-ray computed tomography for taking a tomographic image and the X-ray digital radiography for taking an image of an X-ray transmitted image of an object. The problem of the X-ray computed tomography is that other than a cross-section image of an object cannot be obtained. In the X-ray computed tomographic apparatus, a multiplicity of tomographic images of an object are taken and are connected with each other by image processing thereby to reconstruct an X-ray three-dimensional image. This method, however, has the disadvantage that the multiplicity of tomographic images taken lengthens the total time length of imaging. The X-ray digital radiographic apparatus, on the other hand, can produce only a two-dimensional image.
A radiologic three-dimensional imaging apparatus, as described in "X-Ray Imaging Rotary Radiographic System" by Noboru Niki, Hitoshi Sato, Tikao Uyama and Tatsuo Kumazaki, Medical Imaging Technology, Vol. 10, (1992), pp. 113 to 118, for example, is an X-ray three-dimensional imaging apparatus using an X-ray image intensifier and a conventional video camera as a two-dimensional X-ray image detector. This X-ray three-dimensional imaging apparatus is intended to take X-ray projected images of an object from a multiplicity of directions within so short a time that the movement of the object is ignorable and reconstruct an X-ray three-dimensional image of the object on the basis of the projected images. Generally, however, a two-dimensional radiologic image detector using a conventional video camera for visible image detection poses the problem that the imaging of an object having a large X-ray absorption factor is difficult and objects that can be imaged are limited. This is by reason of the fact that the dynamic range of a conventional video camera is narrow in spite of a wide dynamic range required of a two-dimensional radiologic image detector.
A conventional X-ray three-dimensional imaging apparatus comprises an X-ray image intensifier for X-ray digital radiography combined with a conventional video camera for taking a transmitted image as a two-dimensional X-ray image detector. Generally, however, the use of a two-dimensional X-ray image detector for the X-ray three-dimensional imaging apparatus undesirably allows the scattered X-rays from the object to enter the X-ray image detector.
A radiologic three-dimensional image can be produced by utilizing a plurality of X-ray projected images obtained from an object taken in various directions. A radiologic three-dimensional image is obtained by approximately applying the conventional two-dimensional image reconstruction method to a three-dimensional image reconstruction, collecting data in the three-dimensional radon region or a three-dimensional Fourier region of a three-dimensional image involved, and applying the three-dimensional inverse radon transform or the three-dimensional inverse Fourier transform to the data respectively. This art is well known and is disclosed in the references cited below, for example.
"Practical Cone-Beam Algorithm" by L. A. Feldkamp, L. C. Davis, and J. W. Kress, J. Opt. Soc. Am. A/Vol. 1, No. 6, pp. 612-619 (1984); "Image Reconstruction from Cone-Beam Projections: Necessary and Sufficient Conditions and Reconstruction methods" by Bruce D. Smith, IEEE Trans. Med. Imaging MI-4, pp. 14-25 (1985); "A Reconstruction Algorithm from Helical Cone-Beam SPECT" by Y. Weng, G. L. Zeng and G. T. Gullberg, IEEE Trans. Nucl. Sci. NS-40, pp. 1092-1101 (1993); and "Analytic Reconstruction for Helical-Orbit Cone-Beam SPECT" by Chunwu Wu, Donald L. Gunter, and Chin-Tu Chen, IEEE Nuclear Science Symposium and Medical Imaging Conference (1993).
In the above-mentioned radiologic three-dimensional imaging apparatus, an X-ray source and a two-dimensional radiologic image detector are rotated around an object while a transmitted radiologic image of the object is taken from a multiplicity of directions, so that a three-dimensional image is reconstructed from the resulting transmitted radiologic images. In the process, the object is located at the center of rotation of the X-ray source and the two-dimensional radiologic image detector. As a result, generally, a radio-logically transmitted image of an object taken at a two-dimensional radiologic image detector from a multiplicity of directions is such that the central portion of the object with a high X-ray absorption factor is located at the central portion of the image, and the peripheral portion of the object low in X-ray absorption factor and a region allowing direct X-ray incidence from the X-ray source without being transmitted through the object are located in the peripheral portion of the image.
In this radiologic three-dimensional imaging apparatus, an X-ray beam of low intensity enters the central portion and an X-ray beam of high intensity arrives at the peripheral portion of the two-dimensional radiologic image detector. The difference of the incident radiation dose between the central and peripheral portions of the two-dimensional radiologic image detector corresponds to the dynamic range required of the particular two-dimensional radiologic image detector. In the prior art described above, however, it is difficult to produce a high-quality three-dimensional image corresponding to this wide dynamic range.
Another problem of the prior art is that the contrast of a radiologic image is deteriorated by the X-ray scattered from the object incident into the two-dimensional radiologic image detector.